Add support for protoc plugins

Add a proto.plugin property to allow specifying a custom protoc
plugin to generate the code.

Fixes: 70706119
Test: m am StreamingProtoTest
Change-Id: I1ecdd346284b42bbcc8297019d98d2cd564eb94c

python/python.go

@@ -288,6 +288,8 @@ func (p *Module) hasSrcExt(ctx android.BottomUpMutatorContext, ext string) bool
 }
 
 func (p *Module) DepsMutator(ctx android.BottomUpMutatorContext) {
+	android.ProtoDeps(ctx, &p.protoProperties)
+
 	if p.hasSrcExt(ctx, protoExt) && p.Name() != "libprotobuf-python" {
 		ctx.AddVariationDependencies(nil, pythonLibTag, "libprotobuf-python")
 	}